Features of the all-fiber heavily erbium-doped laser operation in passive Q-switched regime with a repetition rate of pulses up to 700 kHz and pulse duration 70 ns without the use of an additional nonlinear optical elements have been investigated. The generation dynamics of the heavily erbium-doped fiber laser assembled according to the classical Fabry-Perot scheme with two mirrors under continuous direct core pumping at the 976 nm and 1490 nm wavelength was studied. Passive Qswitching in heavily erbium-doped lasers is realized due to effect, which is opposite to the saturable absorption. The absorption grows with the field intensity instead of being saturated. The reason for the appearance of pulses is the existence of ion pairs at high erbium concentrations. This creates an effective absorption dependent on levels’ populations of ion pairs.
In this work, the second harmonic generation was investigated in colloidal solution of CdSe nanoplatelets with two and three monolayers of CdS shell in the case of the two-photon laser excitation of the light-hole – electron transition (1lh-e). In addition to the two-photon photoluminescence spectrum a narrow spectral peak was observed at the wavelength of the laser’s second harmonic radiation, which intensity increases quadratically with incident laser intensity growth taking into account linear absorption of colloidal solution. Also the cause of the second harmonic generation in CdSe/CdS nanoplatelets was considered and explained. It was suggested that the second harmonic in CdSe/CdS colloidal solution is mostly generated by surface atoms for the part of the nanoplatelets satisfying to phase matching condition of the fundamental and second harmonic waves. The concentration of CdSe/CdS nanoplatelets in colloidal solution at which the second harmonic generation can be observed was determined.
Semiconductor nanocrystals have been actively studied due to their unique physical and chemical properties and are actively being implemented in nanophotonics devices. Nanostructures created by the colloidal synthesis with design shape, size and crystal structure are widely used. Recently, colloidal semiconductor quantum wells (nanoplatelets) have been created. Colloidal semiconductor nanoplatelets (NPLs) are atomically flat nanocrystals which demonstrate a zinc blend crystal structure with a [001] axis. Strong quantum confinement of NPLs and high exciton binding energy are provided by anisotropic of nanocrystals with several nanometers thick and tens of nanometers in lateral dimensions which can be used to tune the optical absorption and photoluminescence spectra.
In this paper we present the peculiarities of excitation, interaction and relaxation of excitons in colloidal CdSe nanoplates depend on type and thickness of shell in the case of one-photon exciton excitation by laser pulses (λ=540 nm, τ=10 ns). The linear and nonlinear absorption spectra of colloidal CdSe NPLs were studied. The linear absorption spectrum of the NPLs demonstrate three well-resolved absorption bands that correspond to heavy hole, light hole and spin-orbital exciton transitions at room temperatures due to the almost complete absence size dispersion of nanocrystals. The differential transmission spectra allowed us to reveal experimentally the lowest four band structure of CdSe/CdS nanoplatelets at the Γ point of Brillouin zone and its modification with CdS shell thickness changing for the first time. In addition, the features of exciton-exciton interaction, exciton-phonon interaction, as well as the process of energy transfer between light and heavy excitons to exciton relaxation were investigated. The rate equations describing the exciton-exciton and exciton-electron interactions was applied for analyzing the recombination and interaction of excitons in the colloidal NPLs under high excitation densities.
This work was partially supported by the Russian Foundation for Basic Research №20-32-70001
The features of degenerate four-wave mixing by means of two-photon excitation of exciton transitions in a colloidal solution of CdSe/ZnS quantum dots (QDs) were revealed. This process was studied by means of dynamic Bragg gratings creation in colloidal QDs solution. The intensity of the generated pulses as a result of the degenerate four-wave interaction should be proportional to the cube of the incident pulses intensity. However, the 6th degree dependence and the quadratic dependence were experimentally measured for the first half and for the second half of the pulses in the train, correspondingly. To determine the features of the two-photon absorption the dependence of the intensity of the transmitted and self-diffracted pulses on the incident pulses intensity was measured. The experimental results can indicate a simultaneous effect on the transmission and refraction of the colloidal solution of QDs in the case of resonant two-photon excitation of the basic exciton transition as a non-inertial (classical) nonlinearity and resonance nonlinearity associated with a strong exciton absorption.
Photoluminescence (PL) features and nonlinear transmission of Cu-doped CdSe colloidal quantum dots (QDs) under nanosecond laser pulses excitation were investigated. Strong difference of the pump intensity dependent behavior of basic exciton transition and Cu dopants associated PL was revealed. The presence of exciton-phonon interaction and its strong influence on the PL and nonlinear properties of Cu-doped colloidal CdSe QDs are proved by the simultaneous linear growth of basic excitons PL, the growth of the Stokes shift and significant decrease of absorption at the basic exciton transition wavelength with the increase of pump intensity [1].
The features of the nonlinear absorption of CdSe/CdS core-shell nanocrystals based on 5 monolayer (ML) CdSe nanopletelets (NPLs) in the case of one-photon excitation of the exciton transitions by means of ultra-short laser pulses (non-stationary regime) were investigated. CdSe NPLs were synthesized by colloidal method at the temperature of 210 oC. Optical absorption spectrum of as prepared CdSe NPLs showed narrow excitonic absorption bands at 463 and 436nm corresponding to hh-e and lh-e, respectively, which indicates that the CdSe NPLs have 5ML thickness. The CdSe/CdS core-shell NPLs were obtained using method of colloidal atomic layer deposition (c-ALD). The c-ALD method allows obtaining core-shell NPLs with thickness control at the atomic monolayer level. The obtained CdSe/CdS core-shell NPLs showed narrow and pronounced hh-e and lh-e transitions characteristic for cadmium chalcogonide NPLs, which indicates their high uniformity in terms of thickness. Resonant excitation of heavy hole and light hole excitons was realized for 5CdSe/CdS, 5CdSe/2CdS, correspondingly, and non-resonant excitation both heavy hole and light hole excitons was carried out for 5CdSe/3CdS NPLs.
Excitation of colloidal solution of NPLs was carried out by the second harmonic of passively Q-switched Nd3+:YAG laser (2w, λ=532 nm, the pulse duration is 30 ps). Nonlinear transmission spectra evolution was measured while changing pumping intensity. The variation of excitation intensity was realized by neutral optical filters. The increase in transmission of the exciton transitions at the excitation wavelength was observed for all three samples. This feature of nonlinear change in transmission is attributed to phase space filling effect. The greater induced bleaching was discovered for resonantly excited sample. The saturation intensity of all samples were measured about 50 MW/cm2 for non-stationary excitation regime. The transmission increases in absolute value ΔT=T-T0≈30%, with relative change in transmission ΔT/T0≈50% in the case of resonant excitation of excitons, and ΔT≈15%, ΔT/T0≈35% in the case of resonant excitation. The role of up-conversion and down-conversion processes were defined.
The monocrystalline Bi2Te3-xSex and Bi2-xSbxTe3-ySey films of various compositions and thickness were synthesized by MOCVD on sapphire substrates1 . The transmission of laser pulses (duration τ≈35 ps, wavelength λ=1064 nm) through the films was measured as a function of radiation intensity with the aim to optimize functionality of these films deposited on different substrates as saturable absorbers for the application in passively mode-locked 1-2 μm wavelength lasers. There was investigated nonlinear absorption of films of various compositions and thickness with measured saturation intensity ≈ 20 MW/cm2 for Bi2Te3-xSex and 45 MW/cm2 for Bi2-xSbxTe3-ySey. Data obtained can be explained by the classical nonlinearity – phase space filling in a quantum well2 . As it was established earlier3 , the semiconductor conductivity type of the film without light interaction can be changed to the metallic conductivity type in the case of light interaction with the film, that is strongly nonlinear depends on the light intensity. At the same time, with a metallic conductivity type, the film becomes vulnerable for high light intensity.
In the present paper we analyze peculiarities of CdSe/ZnS colloidal quantum dots (QDs) nonlinear optical properties due to the one- and two-photon excitation of basic electron-hole (exciton) transition by means of ultra-short laser pulses. We also revealed the main mechanisms of optical nonlinearities which result in the laser beam divergency. To find out the role of quantum dots, two different types of solvents were investigated - toluene or hexane based colloidal solutions.
We have investigated the nonlinear absorption of CdSe-based nanoplatelets (NPLs) with different thicknesses of shell in the case of resonant one-photon stationary excitation of exciton transitions by nanosecond pulses of mode-locked Nd:YAP laser. Decrease in absorption at the wavelength of whether light hole – electron and heavy hole – electron exciton transitions was revealed. Induced changing of both absorption doublet components was attributed to phase space filling effect and exciton energy conversion mechanism.
The high-intensity nanosecond laser pulses scattering in strongly absorbing colloidal solutions of СdSe/ZnS quantum dots has been investigated. Different types of nonlinear Tyndall scattering mechanism was revealed as a function of excitation radiation intensity. At the low laser pulses intensity (up to 15 MW/cm2 ) saturation of the basic exciton transition in strongly absorbing colloidal solution of СdSe/ZnS quantum dots was observed. In this case of average laser pulses intensity (15-200 MW/cm2 ) the dominant scattering mechanism is scattering on dipoles induced by the electric field and scattering on density fluctuations of the dispersed medium around bleached quantum dots. At the higher intensity (200-4000 MW/cm2 ), the predominant scattering mechanism is the scattering on bubbles of gas formed around local heating centers – colloidal quantum dots.
The goal of the work is modeling and development of nondestructive method for the doped semiconductor layer
diagnostics and measurement of the impurity levels depth relatively to the conduction band. To carry out diagnostics
for materials with a high linear absorption there is required a method allows to measure material characteristics on the
surface layer. To solve this problem was chosen reflected degenerate four-wave mixing technique. Nonlinear response
increases dramatically in the case of the resonant excitation of electron-hole transition. Reflected degenerate four-wave
mixing has been discovered in the case of one-photon resonant excitation of the excitons (electron – hole) transition for
the atomic-like model structure (highly absorbing colloidal solution of CdSe/ZnS quantum dots (QDs)) by powerful
beams of mode-locked laser with picosecond pulse duration. Formation of the beams in forward direction can be
explained both self-diffraction of the input beams at the induced one-dimensional photonic crystal (induced diffraction
grating) and by forward degenerate four-wave mixing. Backward direction beams formation can be explained only by
reflected degenerate four-wave mixing.
A simple way to create dynamic photonic crystals with different lattice symmetry by interference of non-coplanar laser beams in colloidal solution of quantum dots was demonstrated. With the proposed technique we have made micro-periodic dynamic semiconductor structure with strong nonlinear changing of refraction and absorption and analyzed the self-diffraction processes of two, three and four non-coplanar laser beams at the dynamic photonic crystal (diffraction grating) with hexagonal lattice structure. To reach the best uniform contrast of the structure and for better understanding of the problems, specially raised by the interference of multiple laser beams theoretical calculation of the periodic intensity field in the QDs solution were performed. It was demonstrated that dynamic photonic crystal structure and even it’s dimension can be easily tuned with a high speed by the laser beams polarization variation without changing the experimental setup geometry.
One-dimensional dynamic photonic crystal was formed by a periodic spatial modulation of dielectric permittivity induced by the two ultrashort laser pulses interference in semiconductor quantum dots CdSe/ZnS (QDs) colloidal solution intersecting at angle θ. The fundamental differences of dynamic photonic crystals from static ones which determine the properties of these transient structures are the following. I. Dynamic photonic crystals lifetimes are determined by the nature of nonlinear changes of dielectric permittivity. II. The refractive index changing is determined by the intensity of the induced standing wave maxima and nonlinear susceptibility of the sample. We use the pump and probe method to create the dynamic one-dimensional photonic crystal and to analyze its features. Two focused laser beams are the pump beams, that form in the colloidal solution of quantum dots dynamic one-dimensional photonic crystal. The picosecond continuum, generated by the first harmonic of laser (1064 nm) passing through a heavy water is used as the probe beam. The self-diffraction of pumping beams on self induced dynamic one-dimensional photonic crystal provides information about spatial combining of laser beams.
The goal of this work is the investigation of optical spectra features of zinc selenide (ZnSe), silver iodide (AgI) and its two-phase composite AgI-ZnSe nanostructures produced by laser ablation method, which can be used to design optical sensors and diffractive structures in integrated optics. Shifted to blue wavelengths relatively to the bulk semiconductor material band edge transmission spectra minima have been discovered for the ZnSe and AgI-ZnSe films. The observed minima of the transmission spectra are peculiar to the quantum energy spectra of semiconductor nanostructures. Discovered transmission spectra minima for the ZnSe and AgI-ZnSe films shifted to the short-wavelength region from the energy of the bulk material band gap can be the evidence of nanocrystals formation during the film growth by laser ablation, and which are characterized by the energy spectrum quantization and lower electron and upper hole quantum confinement levels shifts from the bottom of the conduction and the top valence bands, respectively.
Two optical systems modeling of laser and broadband radiation focusing, that is necessary for realization of the pump and probe method, was carried out in this work. Modeling was utilized to construct experimental setup for transmission spectra measuring of studied sample by probe nanosecond broadband radiation (coumarin photoluminescence) depending on the intensity of the nanosecond laser pump pulses. The saturation effect of absorption and the induced charge Stark-effect coexistence and predominate issue of these effects are determined by power of optical excitation. In dependence of tuning of excitation radiation frequency from basic exciton transition frequency nonlinear effects in colloidal CdSe/ZnS quantum dots has been investigated.
We investigate a simple way to create dynamic photonic crystals with different lattice symmetry by interference of four non-coplanar laser beams in colloidal solution of CdSe/ZnS quantum dots (QDs). The formation of dynamic photonic crystal was confirmed by the observed diffraction of the beams that have excited photonic crystal at the angles equal to that calculated for the corresponding three-dimensional lattice (self-diffraction regime). Self-diffraction from an induced 3D transient photonic crystal has been discovered in the case of resonant excitation of the excitons (electron – hole transitions) in CdSe/ZnS QDs (highly absorbing colloidal solution) by powerful beams of mode-locked laser with picosecond pulse duration. Self-diffraction arises for four laser beams intersecting in the cell with colloidal CdSe/ZnS QDs due to the induced 3D dynamic photonic crystal. The physical processes that arise in CdSe/ZnS QDs and are responsible for the observed self-action effects are discussed.
Modeling of self-diffraction pattern formation from the induced diaphragm, arising in the case of the transparency channel saturation by one-photon resonant non-stationary excitation of the basic exciton transition in colloidal quantum dots (QDs) is realized. The simulation results allow us to obtain the reference image of self-diffraction pattern and dependence of the intensity transverse distribution of the output beam from the intensity of the excitation beam, forming a transparency channel. A powerful laser pulse creates a transparency channel, so that it self-diffracts on the induced diaphragm. The possibility to apply the obtained simulation results for intensity estimation of the laser radiation and for the possible application in the technique (nonlinear-optical limiters of intense laser radiation in the visible and nearinfrared region, optical switches) are discussed.
The features of nonlinear and electro-optical processes has been discovered in the case of two-photon resonant excitation of the excitons in colloidal CdSe/ZnS quantum dots. Self-diffraction arises for two laser beams intersecting in the cell with colloidal CdSe/ZnS quantum dots (QDs) due to the dynamic phase grating formatting. The calculated induced change in the refractive is sufficient to form a phase diffraction grating. Such a large value of χ(3) as compared to the third-order nonlinear susceptibility for the solvent (hexane) is due to the increase in χ(3) occurring when the intermediate resonance is attained in a medium transparent for laser radiation. In order to identify physical processes responsible for the induced grating formation and the diffraction efficiency self-diffracted pulse intensity dependences on the incident pulse intensity were measured for two samples of colloidal QD CdSe/ZnS, which frequency of the fundamental exciton transition is tuned to the high-frequency and low-frequency region from the double laser frequency. The discovered cubic dependence of the self-diffracted pulse intensity on the incident pulse intensity was explained by four-wave mixing process. Discovered above 5-th index of power dependence of the self-diffracted pulse intensity on the excitation pulses intensity we explained by the increasing magnitude of two-photon absorption (due to shifting of two photons energy of laser radiation to the exact exciton absorption resonance by red Stark shift of the exciton absorption), accompanied by the growth absorption by two-photon excited carriers that leads to the induced amplitude grating formation in addition to the phase grating.
Two-dimensional (2D) dynamic photonic crystal regime has been utilized to investigate self-diffraction effect and
nonlinear optical properties of excitons in CdSe/ZnS colloidal quantum dots (QDs). Self-diffraction at 2D photonic
crystal arises for three intersecting beams of Nd+3:YAG laser second harmonic in the case of one-photon resonant
excitation of the exciton (electron - hole) transition QDs. The relaxation time of excited excitons has been measured by
pump and probe technique at induced one-dimensional transient diffraction grating. Two-exponential decay with initial
fast and slow parts was discovered. Self-action effect has been discovered in the case of stationary resonant excitation of
excitons in CdSe/ZnS QDs by the beam of second harmonic of powerful 12-nanosecond laser pulses. The bleaching of
exciton absorption and the creation of transparency channel (this effect provokes self-diffraction of the second harmonic
beam) was explained by the dominating coexisting and competing processes of state filling in stationary excited quantum
dots and Stark-shift of exciton spectral band. The peculiarities of the influence of these processes at the change of
exciton absorption in quantum dots in the case of different detuning from exciton resonance (quantum dots with different
size have been used) was analyzed.
Self-diffraction of two types has been discovered in the case of resonant excitation of excitons in CdSe/ZnS quantum dots (highly absorbing colloidal solution) by powerful beams of mode-locked laser with picosecond pulse duration. I. The bleaching of exciton transition provokes the creation of transparency channel and laser beam’s self-diffraction at the induced circular aperture. II. Self-diffraction arises for two laser beams intersecting in the cell with colloidal CdSe/ZnS quantum dots due to the induced transient diffraction grating. Nonlinear optical properties responsible for the observed self-action effects in CdSe/ZnS quantum dots are discussed and a method for estimating laser pulse duration is suggested.
The anomalous nonlinear absorption of colloidal CdSe/ZnS quantum dots has been revealed in the case of resonant
excitation of the basic exciton transition by powerful picosecond light pulses: bleaching at moderate input intensities is
substituted by retardation of bleaching rate and even by increasing of absorption. The latter has been explained by
saturation (state-filling) effect of two-level system with decreasing lifetime of the excited state at high excitation because
of Auger recombination. The observed changes of output intensity cross-section profile versus exciting intensity were
attributed to the creation of transparency channel in the cell with highly absorbing colloidal solution of quantum dots by
laser beam due to state-filling, strip-effect and finally self-diffraction of Fresnel or Fraunhofer type by induced aperture.
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